MH370 airliner MYSTERY: The El Reg Pub/Dinner-party Guide

Some tech angles for your consideration

0107 local time 8 Mar (1707 GMT 7 March)

As MH370 crosses the Malaysian coast headed out over the South China Sea, the final Aircraft Condition Monitoring System (ACMS) report is transmitted from the plane via the Aircraft Communications Addressing and Reporting System (ACARS) data link. This is an automated report containing a variety of information from systems aboard the plane, relayed to various destinations on the ground via VHF radio or (when out of range, as when over the sea) by satellite communications.

The data reporting - and indeed the aircraft's satellite communications for all purposes - was disabled via processes that would have required navigating through some menus on the flight deck and selecting options on a keypad. Most 777 pilots would be able to do this, much though it isn't something they would ever normally do.

However the ACARS satellite communications equipment was not, in fact, completely disabled. The plane's satellite transmitter continued to automatically send short radio "blips" each hour. These contained no data as such: the purpose of the transmissions was to inform the satellite up above that there was an active transmitter there which might send a message at some point. The satellite would thus know to keep one of its receiving assemblies aligned at a suitable angle.

It would be possible to turn off this transmission also, but that would mean getting into an electronics bay below the cockpit. Our pilot source didn't even know that such transmissions took place until all this happened, though he thought that if he had known he could get into the bay via a locked floor hatch (actually in the forward galley on the 777s he had flown) and cut power altogether to the satcomms equipment. He adds that if someone other than the pilots gained access to the electronics bay and interfered with comms kit, alarms and notifications would have appeared on the flight deck, so it's all but certain that the pilots were involved in the process - whether under coercion or not.

0119 local 8 Mar (1719 GMT 7 March)

As the aircraft is handing over from Malaysian to Vietnamese traffic control, the final voice transmission is received in Malaysia - apparently sent by the co-pilot - saying that all is well and "good night". The flight never contacts Vietnamese air controllers, and at the same time the aircraft's secondary-radar transponder is switched off.

Background: Most aircraft are fitted with secondary-radar transponders which, when they detect the emissions from an air-traffic radar on the ground, send a response signal which includes a "squawk" code. The controller sees on his or her screen not only a dot showing the plane's location but also the identifying code, greatly simplifying his or her task. Without benefit of transponders, civilian air traffic radars are essentially useless, especially at long ranges. MH370 thus disappeared from civilian ATC screens at this point.

However a military air defence radar is designed to work without benefit of transponders, detecting an aircraft purely by its own transmitted radio energy reflected back off the plane's body (the so-called "skin paint", which stealth-fighter designers strive so hard to eliminate). Flight MH370 was still at this point being tracked by Malaysian air defence radar, and later reconstruction of military radar logs shows that the airliner - having turned off its transponder - then swung round away from China to head due west. The Malaysian air force tracked it as it flew right across the Malaysian peninsula, partly over their territory and partly over Thailand. (The fact that a large unidentified aircraft could do this without the military radar operators apparently caring enough to notify anyone is now causing a certain amount of trouble for Malaysian air-defence commanders.)

The military radars apparently detected various changes in height, but it should be noted that the ability of a long-range search radar to tell what height contacts are at is usually pretty poor. Other strange height data have supposedly been established from early ACMS engine data reports, but these aren't being given a lot of credence either.

Malaysian military radar loses contact with flight MH370 over the Andaman Sea, headed outbound in the direction of the Indian Ocean.

The extent and effectiveness of a country's air-defence radar coverage is normally a military secret, so other countries in the region are naturally reticent about whether they did (or could) detect the rogue airliner at any stage. So the last publicly known position of MH370 is when the Malaysian air force lost it.

However, almost certainly unknown to whoever was directing the airliner's course, its satcomms equipment was still sending its automated blips each hour to a geostationary communications satellite orbiting high above the Indian Ocean. Each blip was logged, and subsequently experts were able to use the records to generate an estimate of what range the aircraft was at from the satellite at that point. (In theory this would mean the satcomms transmitter could be anywhere on large sections of a sphere around the spacecraft, but assuming that the plane is within the Earth's atmosphere allows a circle to be drawn on the map.)